An electric power steering apparatus (EPS) which is equipped with the motor control unit, and provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus (EPS) will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a pinion-and-rack mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (an electronic control unit (ECU)) 30 for controlling the electric power steering apparatus from a battery 13 as a power supply, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command (a steering assist command) on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Ve1 detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the calculated current command value. A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor which is connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Ve1 from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a control section that includes an MCU (including an CPU and an MPU), and general functions performed by programs within the control section are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Ve1 from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Ve1 using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result ΔI (=Irefm−Im) in the subtracting section 32B is proportional-integral-controlled (PI-controlled) at the PI-control section 35. The voltage control value Vref obtained by the current control is inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a self-aligning torque (SAT) detected or estimated and an inertia compensation value 342 at an adding section 344. The addition result is further added with a convergence control value 341 at an adding section 345. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the characteristics of the current command value Iref1.
In a case that the motor 20 is a three-phase brushless motor, details of the PWM-control section 36 and the inverter 37 have a configuration as shown in FIG. 3, and the PWM-control section 36 comprises a duty calculating section 36A that calculates the PWM duty values D1 to D6 which are used in a three-phase PWM-control by using the voltage control command value Vref in accordance with a predetermined equation, and a gate driving section 36B that drives gates of the FETs as the driving device by means of the PWM-duty values D1 to D6 and turns-ON or turns-OFF the gates of the FETs for compensating a dead time. The modulation signal (carrier) CF is inputted into the duty calculating section 36A, and the duty calculating section 36A calculates the PWM-duty values D1 to D6 by synchronized to the modulation signal CF. The inverter 37 is configured to the three-phase bridges of the FETs. The motor 20 is driven by turning-ON or turning-OFF the respective FETs by using the PWM-duty values D1 to D6.
A motor release switch 23 is interposed between the inverter 37 and the motor 20 in order to block a current supply when the assist control is stopped and the like. The motor release switch 23 comprises the FETs with parasitic diodes disposed to respective phases.
Recently, redundancy of the steering system is required, and the motor having multi-system motor windings is used to the motor for the assist-control. For example, FIG. 4 shows a star (Y)-connection of the three-phase motor. One system comprises a U-phase winding UW1, a V-phase winding VW1 and a W-phase winding WW1, and the other system comprises the U-phase winding UW2, the V-phase winding VW2 and the W-phase winding WW2. The motor is driven by passing the three-phase currents through the windings UW1 to WW1 or the windings UW2 to WW2. FIG. 5 shows a delta (A)-connection of the three-phase motor. One system comprises the U-phase winding UW1, the V-phase winding VW1 and the W-phase winding WW1, and the other system comprises the U-phase winding UW2, the V-phase winding VW2 and the W-phase winding WW2. The motor is driven by passing the three-phase currents through the windings UW1 to WW1 or the windings UW2 to WW2.
The motor 120 having such the multi-system windings (dual-system windings) is driving-controlled by, for example, the dual-system driving control systems (the MCU, the microcomputer and the like), as shown in FIG. 6.
That is, the overall control is performed by the MCU 100. The first system windings #1 of the motor 120 having the dual-system motor windings are driving-controlled by the inverter 121A through the motor release switch 122A, and the second system windings #2 are driving-controlled by the inverter 121B through the motor release switch 122B. The MCU 100 controls turning-ON or turning-OFF of the FET1A to the FET6A of the inverter 121A through the gate driving section 140, and controls turning-ON or turning-OFF of the FET1B to the FET6B of the inverter 121B through the gate driving section 140. The electric power is supplied from the battery 150 to the inverter 121A and the inverter 121B.
In such an electric power steering apparatus of the dual-system control, for example, as shown in Japanese Patent No.4998836 B2 (Patent Document 1), the reverse-connection protection-FETs are disposed to respective systems not to pass the overcurrent through the ECU and be burned out the ECU when the battery is reverse-connected. That is, in the Patent Document 1, the dual-system reverse-connection protection-FETs are disposed for the reverse-connection protection of the battery. In a case that the FET driving circuit (a pre-driver) in one system is failed, the dual-systems have a configuration that the reverse-connection protection-FETs and the inverter in the other system continue operating.